Field of the Invention
Embodiments of the invention relate to an organic light emitting diode display and a method for sensing driving characteristics thereof.
Discussion of the Related Art
Because an organic light emitting diode display is a self-emission display device, the organic light emitting diode display may be manufactured to have lower power consumption and thinner profile than a liquid crystal display requiring a backlight unit. Further, the organic light emitting diode display has advantages of a wide viewing angle and a fast response time. As the development of a process technology reaches a large-sized screen mass production technology, the organic light emitting diode display has expanded its market while competing with the liquid crystal display.
Each of pixels of the organic light emitting diode display includes an organic light emitting diode (OLED) having a self-emitting structure. Organic compound layers including a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, an electron injection layer EIL, etc. are stacked between an anode terminal and a cathode terminal of the OLED. The organic light emitting diode display implements an input image using a phenomenon, in which the OLED emits light when electrons and holes are combined in an organic layer through a current flowing in a fluorescence or phosphorescence organic thin film.
The organic light emitting diode display may be variously classified depending on kinds of emission materials, an emission method, an emission structure, a driving method, etc. The organic light emitting diode display may be classified into a fluorescent emission type and a phosphorescent emission type depending on the emission method. Further, the organic light emitting diode display may be classified into a top emission type and a bottom emission type depending on the emission structure. Further, the organic light emitting diode display may be classified into a passive matrix OLED (PMOLED) display and an active matrix OLED (AMOLED) display depending on the driving method.
Each pixel of the organic light emitting diode display includes a driving thin film transistor (TFT) controlling a driving current flowing in the OLED depending on data of the input image. Driving characteristics of the pixels have to be the same as one another at all of locations of the screen. However, the driving characteristics of the pixels may vary depending on the location of the screen due to a process deviation. Further, the driving characteristics of the pixels may vary depending on a driving time and a driving environment. Examples of the driving characteristic of the pixel include a threshold voltage of the OLED, a threshold voltage of the driving TFT, and a mobility of the driving TFT.
An external compensation technology for sensing the driving characteristics of the pixels and compensating for the driving characteristics using a driving circuit outside a display panel has been proposed as a method for increasing the image quality and the lifespan of the organic light emitting diode display.
The external compensation technology senses the driving characteristics of the pixels based on changes in the anode voltage of the OLED or changes in a source voltage of the driving TFT using an analog-to-digital converter (ADC) and modulates data, thereby compensating for changes in the driving characteristics of the pixels. The ADC is designed in consideration of an estimated range of changes in the driving characteristics of the pixels due to the degradation of the driving characteristics, the size of an integrated circuit (IC) in which the ADC is embedded, the sensing accuracy, a sensing scale, and the like. A sensing circuit including the ADC may accurately sense driving characteristic of a pixel, which will be firstly examined, in an initial sensing environment. However, when the driving characteristics of the pixels greatly change because of the elapse of driving time and changes in the driving environment of the pixel, the driving characteristics of the pixels cannot be accurately sensed. This is because output data of the ADC overflows when the changes in the driving characteristics of the pixels are outside the range (hereinafter, referred to as “sensing range”) of an input voltage, which can be accurately sensed by the ADC. The ADC outputs all of the voltages exceeding the sensing range as digital data of a maximum value.
For example, when the sensing range of the ADC is 2V and the ADC outputs 10-bit digital data, the ADC converts a range (for example, 1V to 3V) of 2V into digital values of 1024 stages. However, when the anode voltage (or the threshold voltage) of the OLED is 4V, the anode voltage of the OLED exceeds the sensing range of the ADC. Therefore, the ADC outputs the digital data value “1024” corresponding to “2V”. As a result, the anode voltage of the OLED is sensed as 2V, and the driving characteristic of the pixel is inaccurately sensed. Accordingly, when changes in the driving characteristic of the pixel exceed the sensing range of the ADC, the driving characteristic of the pixel is inaccurately sensed.